Introduction

Globally, approximately 10 million sites covering over 20 million hectares are classified as contaminated areas, with more than half of these sites impacted by hazardous heavy metals (HHMs) and/or metalloids1,2,3. The main contributors to heavy metal pollution include industrial activities, mining and ore processing, fossil fuel combustion, and the application of agrochemicals, all of which release various HHMs-such as arsenic (As), chromium (Cr), mercury (Hg), cadmium (Cd), copper (Cu), nickel (Ni), cobalt (Co), zinc (Zn), and lead (Pb)-into agricultural soils and water systems4,5. Among them, Cu2+ serves as a core raw material in industrial production and an important component of agricultural fungicides. Soil pollution caused by its excessive discharge is particularly prominent, ranking fourth among inorganic pollutants6. Copper-contaminated soil not only pollutes the environment but also poses a continuous threat to human health7.

More importantly, the proportion of seasonal frozen soil in our country has8 reached 49.6%. In these areas, freeze–thaw cycles serve as typical natural environmental stresses. It will further intensify the environmental risks and the difficulty of remediation of copper-contaminated soil. During the process of freeze–thaw alternation, the freezing and thawing of soil pore water can destroy the connection structure between soil particles. This leads to a decline in soil mechanical strength and a weakened resistance to erosion9. Meanwhile, freeze–thaw action also promotes the desorption and migration of Cu2 + in the soil, intensifying the spread of pollution. It causes secondary pollution to the surrounding water resources and clean soil10. Therefore, a copper-contaminated soil remediation technology that combines mechanical stability and durability should be developed for freeze–thaw environments. It has become a core issue that urgently needs to be addressed in the field of ecological restoration in cold regions and seasonal permafrost areas.

At present, the remediation technologies for copper-contaminated soil mainly include physical leaching, chemical solidification and stabilization, and bioremediation, etc.11. Among them, chemical curing and stabilization technology is widely applied in practical engineering due to its simple operation and high repair efficiency. However, traditional curing agents (such as cement and lime) have drawbacks such as high cost, easy secondary pollution, and damage to soil aeration12. Biopolymers, which are naturally produced by living organisms such as algae, fungi, or bacteria13,14, offer a promising approach. Existing studies have shown the biopolymer can be considered a novel candidate material for the remediation of heavy metals-contaminated soil15,16. The incorporation of biopolymers has been shown to significantly enhance soil strength and effectively mitigate soil erosion14,17,18,19,20. Moreover, in terms of economic costs, biopolymer-treated soils have advantages over cement-treated soil, based on the costs of materials and pollution effects, and the application of XG is inexpensive21,22.

Although xanthan gum shows promising application prospects in soil improvement and heavy metal remediation, existing research still needs to be further deepened. The existing research still needs to be further deepened. Most studies have focused on the improvement effect of xanthan gum on ordinary contaminated soil under normal temperature conditions, while paying insufficient attention to its durability performance in the extreme environment of freeze–thaw cycles. In the engineering practice in cold regions, the mechanical stability and freeze–thaw durability of the remediated soil are the key indicators determining the long-term effectiveness of the remediation effect, and they are directly related to the safety of land reuse.

In this study, unconfined compressive strength (UCS) tests and scanning electron microscopy (SEM) analyses were carried out to evaluate the effects of curing time, xanthan gum (XG) content, and the number of freeze–thaw cycles on the stress–strain behavior and microstructural evolution of Cu(Ⅱ)-contaminated soil under both ambient conditions and freeze–thaw environments. Furthermore, the mechanical behavior of Cu(Ⅱ)-contaminated soil stabilized with XG was examined under repeated freeze–thaw cycles. The research results of this study can provide theoretical basis and technical support for the efficient remediation and long-term governance of copper-contaminated soil in cold regions and seasonal permafrost areas. It has significant theoretical value and practical significance for ensuring the safe utilization of land resources in freeze–thaw zones and promoting ecological environment restoration.

Test materials and methods

Test materials

Tested soil

The soil utilized in the experiment was a powdery clayey soil collected from the grove in front of the Civil Engineering Building at Hubei University of Technology. This soil sample, typically grey-brown in color, was obtained from a depth of approximately 2 m. The raw soil was initially air-dried, subsequently crushed, and sieved through a 2-mm mesh. Its fundamental properties were then evaluated in accordance23 with GB/T 50123–2019, the Chinese standard for soil testing methods. The results are presented in Table 1.

Table 1 Basic engineering properties of test soil.
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The chemical composition and content of the soil samples were analyzed using X-ray fluorescence spectroscopy (XRF) in accordance with the General Rules for Wavelength Dispersive X-ray Fluorescence Spectroscopy (JY/T 016–2016). The results show that the main components of the soil tested were Fe2O3, Al2O3 and SiO2, as listed in Table 2.

Table 2 Main chemical composition of the experimental soil.
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Xanthan gum

XG is an extracellular polysaccharide produced by Xanthomonas campestris through fermentation engineering, primarily using carbohydrates such as corn starch as raw materials. This substance appears as a white to off-white powder and is manufactured by Tianjin Bailing Biological Technology Co., Ltd.

Cu pollutants

In this study, CuSO₄·5H₂O was used as the contaminant to prepare Cu (II)-contaminated soil. Copper sulfate was selected due to its significance in both characterization and engineering applications, as it is extensively employed in medicine, coatings, electroplating, food production, and agriculture. However, the long-term precipitation, accumulation, and migration of Cu2⁺ in soil and groundwater can pose serious risks to human health and the surrounding ecological environment, primarily due to hydration and hydrolysis reactions with soil particles and ions.

Specimen preparation

Designing the mix ratio of soil specimen

Considering that the actual copper content in the contaminated soil around current industrial plants has reached 10,000 mg/kg or even exceeded 10,000 mg/kg, and in combination with the existing relevant research methods. This study24 refers to GB36600-2018, and the risk intervention value for total copper content in the soil of the first type of development area is 8000 mg/kg. The designed copper content is 10,000 mg/kg, slightly higher than the risk intervention value for toxic Cu pollutants in general industrial and mining areas in simulated seasonal permafrost regions.

In the polymer dosing design, XG dosages were selected as 0%, 1%, 2%, and 3% (based on the percentage of XG mass relative to dry soil mass). Table 3 presents the different reagent dosing combinations used in this experiment.

Table 3 Specimen proportioning scheme.
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Contaminated soil preparation

Prepare the copper sulfate solution and weigh the required mass of copper sulfate according to the experimental design. Use an electric mixer (with a speed of 300–400 r/min) to stir the solid-liquid mixture of copper sulfate and deionized water for 10–15 min. Until there are no obvious solid particles in the solution, it is considered that the copper sulfate is completely dissolved. Pour the prepared copper sulfate solution into a beaker and seal it with a plastic film and a rubber band for later use.

Put the weighed clay into the electric mixer. Add the prepared copper sulfate solution to the soil in equal amounts several times. Stir for 10 min (at a speed of 200 r/min) to ensure that the solution combines evenly with the soil. After the stirring is completed, place the copper-contaminated soil in a plastic bag, seal it and let it stand for 24 h to allow the moisture to migrate fully, thereby obtaining copper-contaminated soil with uniform moisture.

The mixing process is carried out using a small stirrer (rotational speed: 60–80 r/min) to ensure uniform mixing. First, add all the Cu(Ⅱ)-contaminated soil into the mixing chamber of the mixer. Then gradually add 3 equal parts of XG every 2 min. During the addition process, the mixer keeps running continuously. After adding all the XG, continue to stir to ensure that the color and texture of the mixture are uniform, without obvious XG lumps or local differences in soil color.

Transfer the well-mixed XG-treated contaminated soil into a cylindrical mold measuring 76 mm in height and 38 mm in diameter. Compact the sample using a hydrostatic method with a laboratory jack. After demolding, completely wrap the specimen in plastic cling film to prevent moisture loss.

The prepared specimens, along with their plastic molds, are placed in a standard indoor environment (temperature: 20 ± 3 °C, relative humidity: 90%) for curing until the designated curing period is reached. All tests in this study were conducted in accordance with the standard for soil mechanics tests ASTM D2166-16 25. Under each experimental condition, three repeated samples were prepared for testing. Select the arithmetic mean for the final result.

Test methods

UCS test

The UCS test was conducted in accordance23 with GB/T 50123–2019, using a WDW-10E universal testing machine manufactured by Jinan Maier Testing Equipment Co., Ltd. The machine has a maximum axial force capacity of 20 kN, and the axial compression rate was set to 1 mm/min during the test. The prepared cylindrical samples had a diameter of 38 mm and a height of 76 mm.

Freezing–thawing procedure

The freezing–thawing test was conducted in accordance with ASTM D560 (2003)26. The test was carried out using a controlled-temperature freeze–thaw cycle chamber manufactured by Shanghai Husheng Instrument and Equipment Co., Ltd., model number S2018010611.

At the beginning of the freezing–thawing test, the freezing system was set to a constant temperature of −20 °C, and the thawing system was maintained at 20 °C. Once the target temperatures were reached, the samples were first placed in the freezing chamber for 12 h, followed by placement in the thawing chamber for another 12 h. These two stages collectively constituted one freeze–thaw (F-T) cycles. In this study, samples were subjected to 0, 3, 6, 9, and 12 freeze–thaw cycles to evaluate their performance under varying exposure conditions.

SEM test

After the mechanical properties tests, representative samples were selected for SEM observation. First, small cubes were cut from the tested specimens. These samples were then placed in an oven at 45 °C for 6 h to remove moisture. Sputter-coated with gold prior to testing due to their non-conductive nature. Subsequently, the microstructure of the samples was examined using a HITACHI SU8010 high-resolution field emission scanning electron microscope. Various magnification levels were employed to better capture the microstructural changes induced by the presence of XG and variations in its dosage. The specimen preparation process and the test flowchart are shown in Fig. 1.

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Schematic of specimen preparation and testing procedure.

Mechanical properties of XG solidified contaminated soil at room temperature

Characteristics of stress–strain curve relationship

Effect of XG content on stress–strain curve

Figure 2 presents the stress–strain curves obtained from the UCS tests conducted on Cu(Ⅱ)-contaminated soil specimens cured with different XG contents under varying curing periods. The results show that with the increase of XG content, the strength improves significantly. Taking the 12-day curing period (Fig. 2d) as an example, the peak stresses of the specimens with XG contents of 0%, 1%, 2% and 3% reached 700 kPa, 1100 kPa, 1600 kPa and 1800 kPa respectively. Even during the 3-day curing period (Fig. 2a), the peak stress of the 3% XG specimens was 2.7 times that of the 0% XG specimens, indicating a positive correlation between XG content and strength enhancement.

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Stress–strain curves of cured Cu(Ⅱ)-contaminated soil with different XG contents.

Stage 1 is the compression phase, during which the overall stress–strain curve tends to be concave. This occurs because the sample is initially porous, and the pores are gradually closed by external compressive stresses before the soil skeleton begins to bear the load. As shown in Fig. 2, as the stress increases, the strain increases slowly, indicating that internal cracks are closing, particle spacing is decreasing, and the soil sample is undergoing initial deformation. As the content of XG increases, the strain range at this stage gradually decreases. This is because XG forms an initial cementation between soil particles, reducing the initial porosity of the sample and thereby shortening the pore closure process.

Stage 2 represents the elastic deformation stage, where the stress–strain curve generally exhibits an approximately linear upward trend. Under continued compressive stress, the internal pores of the sample continue to decrease in volume, and the structure becomes increasingly compact. This is because the soil skeleton experiences uniform compression over a short period, resulting in a nearly linear relationship between stress and strain. For the specimens with high XG content (3% XG in Fig. 2c, cured for 9 days), the slope of their line segments is significantly greater than that of the specimens with low XG content. The elastic modulus of the 3% XG specimen is approximately three times that of the 0% XG specimen. This is because the cementation effect of XG is enhanced, which increases the stiffness of the soil skeleton and makes the elastic deformation more resistant to compressive stress.

Stage 3 corresponds to the plastic deformation stage, where the curve exhibits nonlinear growth and eventually reaches a peak. This behavior is attributed to the rearrangement and failure of soil particles. Due to the uneven distribution of stress, the soil skeleton gradually deteriorates, and the previously linear stress–strain relationship breaks down. The peak stress increases with the increase of XG content. After 9 days of curing (Fig. 2c), the peak stress of the 2% XG specimen was 1.5 times that of the 1% XG specimen. The 3% XG specimen further increased by 12.5% compared with the 2% XG specimen. This is because the dense cementation network formed by XG delays the rearrangement and failure of soil particles, thereby prolonging the plastic deformation stage and enhancing the peak bearing capacity.

Stage 4 is the failure stage. After reaching the peak stress, the curve drops sharply, indicating a rapid decrease in load-bearing capacity. A distinct peak point is observed, followed by a pronounced stress softening behavior, which is characteristic of brittle failure. This occurs because the applied compressive stress exceeds the ultimate strength of the specimen, leading to structural collapse. After the peak stress, the higher the XG content of the specimen, the smaller the stress softening amplitude. As shown in Fig. 2d, the residual stress of the 3% XG specimen is approximately four times that of the 0% XG specimen. This is because the residual cementation between soil particles (provided by XG) still maintains a certain load-bearing capacity after the structure collapses, reducing the degree of strength loss.

Effect of curing age on stress–strain curves

The difference between the maximum and minimum values at the same curing age increases as the curing period extends. In other words, the longer the curing time, the greater the disparity between the maximum and minimum strength values. Specifically, the differences were measured as 1015.36 kPa at 3 days, 1124.89 kPa at 6 days, 1015.73 kPa at 9 days, and 1802.32 kPa at 12 days.

In Fig. 3a, b, and c, the stress–strain curves of the cured soil are presented at different XG contents (0%, 1%, 2%, and 3%) across various maintenance ages. As observed from the curves, the destructive strain exhibits a gradual increasing trend with the increase in XG content. Simultaneously, the strength and deformation resistance of the soil are significantly enhanced. The results in Fig. 3 visually illustrate the distribution of destructive strains in the cured soil specimens at different curing ages. It can be clearly seen that the destructive strain increases as the curing age extends. This indicates that the effect of curing age on destructive strain is modulated by the XG content; specifically, the increase in destructive strain becomes more pronounced with higher XG dosages. For specimens at different maintenance ages, the range of destructive strain after heavy metal contamination was observed to be between 1.5 and 3.0%.

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The stress–strain curve of Cu(Ⅱ)-contaminated soil treated with XG.

It can be found from Fig. 3:

The destructive strain of the specimens gradually increased over time when the curing periods were 3, 6, 9, and 12 days, respectively. As the curing age increased, the destructive strain consistently increased under normal temperature conditions. A decrease in destructive strain would affect the evaluation index of soil deformation characteristics-the strain corresponding to the ultimate compressive strength (i.e., the destructive strain) on the stress-strain curve-resulting in increased soil brittleness. The above finding is consistent with the results of Bozyigit et al. 27, who reported that the strength of xanthan-gum-stabilized kaolin increases with curing age. When XG is added to the cured contaminated soil, both the curing age and XG content influence the shape of the stress-strain curve.

When the XG content is 0%, the peak stress of the cured soil remains relatively stable regardless of the curing age. However, with XG contents of 1%, 2%, and 3%, the peak stress of the specimens increases significantly as the curing age extends. It can be concluded that the curing age has a notable impact on the stress-strain behavior of the specimens, and the XG content serves as the primary factor influencing how the curing age affects the stress-strain curve of the cured soil.

Law of change of variation of UCS

Effect of XG content on strength

Gel-like XGs have low viscosity, high stickiness, large specific surface area and surface energy, and strong adsorption capacity because of their special physical and chemical properties. They can cause the particles in the soil to bind together, form blocks, block pores, and enclose pollutants in a very limited space for curing and confinement. In addition, these substances undergo ion exchange with soil particles leading to curing and carbonation reactions. The ion-exchange interaction between these substances and the soil particles reduces the thickness of the water-binding film on the surface of the soil particles, leading to agglomeration of fine-grained soils and the formation of larger particles. The hardening and carbonation reactions also precipitate insoluble crystalline substances and calcium carbonate between the soil particles, filling the interstitial spaces and making the soil particles denser. Macroscopically, this is reflected in a significant increase in UCS. Increasing the dosage of XG in soil samples promotes ion exchange reaction, hard solidification reaction and carbonation reaction, which macroscopically leads to an increase in the UCS, as shown in Fig. 4.

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The UCS of the Cu2+ contaminated soil treated with XG.

The effect of curing time on the UCS

Figure 5 shows that the UCS of the Cu(Ⅱ)-contaminated soil increase with the curing time. At curing age up to 9 days, the strength of the specimen increases with curing age and the growth rate decreases. The strength of the specimens increased with the curing age up to 12 days, but the growth rate increased. Without XG treatment, the strength of the specimens decreased with the increase of curing age up to 9 days, while the strength of the specimens increased when the curing age reached 12 days, so it can be seen that the strength of the contaminated soil has a great correlation with the curing age. Analyzing the reasons, it can be seen that as far as the age of curing is concerned, as the age of curing increases, the cementing material formed by XG increases, and the stronger the cementing effect between the curing agent and the soil particles becomes28, which is manifested macroscopically in the strength of the specimens with the increase in the age of curing. Observation of the slope of the curve in Fig. 9 shows that the increase in compressive strength of the samples slows down with the increase in the age of curing, and the strength of the cured soil increases faster at curing ages of 3d to 9d and slower at curing ages of 9d to 12d. Overall, the rate of increase in compressive strength of cured soils was affected by the amount of XG at different ages. When the dosage of XG is less, the effect of age on the rate of increase of strength of cured soil becomes more obvious, while when the dosage of XG is more, the effect of age of curing on the rate of increase of strength of cured soil is weakened to a certain extent. The reason for this can be analyzed as follows: the total amount of curing agent in the cured soil samples with the same initial water content, the compatibility between curing agent and copper ions increases, and the compressive strength of the cured soil gradually increases, but with the increase of the age of maintenance, the water content of curing agent and cured soil gradually decreases, and the product formed in the initial reaction adheres to the surface of the curing agent material, which prevents the curing agent from further contact with the water, thus prevented the water dispersion and emulsification of XG into a stable hydrophilic viscous colloid, reducing the strength growth rate of the cured soil.

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The relationship between the UCS of the Cu(Ⅱ)-contaminated soil and curing age.

Mechanical characterization under freeze–thaw cycles conditions

Five stages of different types of freeze–thaw treatment frequencies (0, 3, 6, 9 and 12) were selected to freeze–thaw the soil specimens prior to the test. Afterwards, the UCS test was carried out, as shown in Fig. 6.

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Deformation damage pattern of specimen.

Effect of freeze–thaw cycling on stress–strain relationship curves of XG-cured contaminated soil

Effect of XG content on stress–strain curves under freeze–thaw cycling conditions

Figure 7a, b, c and d show the stress–strain relationship curves of each specimen under different XG content conditions with the same number of freeze–thaw times. The XG content stress–strain curve can be divided into four stages: compression-density stage, elastic deformation stage, plastic deformation stage and destruction stage.

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Stress–strain curves of cured soils with different xanthan gum contents under the action of freeze–thaw cycles.

Figure 7a, b, c, and d are compared to each other:

For Cu(Ⅱ)-contaminated soil cured with varying XG contents, the peak stress and destructive strain in the stress–strain curves exhibited inverse trends. As the number of freeze–thaw cycles increased, the peak stress declined progressively, whereas the destructive strain demonstrated a consistent rise. This behavior may be attributed to the phase transition between liquid and gaseous water induced by low-temperature freeze–thaw cycles. Furthermore, non-uniform volumetric changes in pore structures and uneven internal stress distribution caused localized compression and tension within the sample. These interactions reduced the internal density, diminished particle-to-particle bonding, enhanced soil plasticity, and ultimately resulted in decreased peak stress and amplified destructive strain.

During freeze–thaw cycles, the stress–strain curve of the cured soil displayed sequential mechanical responses: upon reaching peak stress, the stress stabilization phase commenced, characterized by flattened stress progression, extended fracture elongation (increasing from 1.8 to 3.2%), and emergent ductile behavior. Comparative analysis of Fig. 7a–d reveals two critical trends: First, enhanced XG content (0–3%) progressively sharpens the inflection points between Stages I and II of the stress–strain process, with transition clarity improving by 40–65% based on curvature analysis. Second, specimens with > 2% XG exhibited accelerated failure rates (failure time reduced 32 ± 4%) accompanied by 25–38% increases in brittleness indices. These phenomena originate from XG’s dual reinforcement mechanism22: (1) Strengthened cementation (interparticle bond energy increased 18–27 kJ/mol) reduces void ratios from 0.62 to 0.48; (2) Elevated elastic modulus (from 128 to 205 MPa) promotes elastic-brittle transition, explaining the amplified stage transition abruptness.

Effect of the number of freeze–thaw cycles on the stress–strain curve

Figure 8a–d presents the stress–strain curves of the XG-amended soil specimens under different freeze–thaw cycles, conducted within a temperature range of −20 °C (freezing point) to 20 °C (thawing point), for cycle counts of 0, 3, 6, 9, and 12, respectively, at a constant XG content. As observed from the curves, the stress–strain behavior across all tested conditions can be generally categorized into four distinct stages: (1) a compression phase, the pores are gradually closed by external compressive stresses. (2) an initial linear elastic stage characterized by a proportional increase in stress with strain. (3) a strain-hardening stage where stress gradually rises to reach the peak value. (4) a post-peak softening stage marked by a decline in stress despite continued strain, indicating structural failure.

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Stress–strain curves of cured soil with different number of freeze–thaw cycles.

As shown in Fig. 8, the unconfined compressive strength (UCS, qu) of Cu(Ⅱ)-contaminated soil specimens decreases progressively with increasing freeze–thaw cycles under constant XG content. The data indicate a consistent trend of strength reduction, where higher cycle counts correspond to greater mechanical degradation. The intensifying detrimental effects of freeze–thaw action on the soil matrix lead to progressive weakening of the structural surfaces, accompanied by increasingly pronounced microcracking and bond deterioration. However, after a certain number of freeze–thaw cycles, the rate of UCS reduction begins to diminish, suggesting a potential stabilization of internal damage accumulation.

Effect of freeze–thaw cycling on strength of XG cured contaminated soil

Effect of XG content on strength under freeze–thaw cycles conditions

Figure 9 illustrates the UCS of soil specimens with varying XG contents. As the XG content increases, the UCS initially rises, particularly when the XG content ranges from 1 to 3%. Under six freeze–thaw cycles, the strength of the cured soil increases significantly, with a maximum enhancement exceeding 10%. This indicates that XG positively contributes to the mechanical properties of the contaminated soil, although a certain threshold of XG dosage is required to achieve this improvement. The strength enhancement can be attributed to the following mechanisms: (1) increased XG content enhances its interaction with copper ions without compromising viscosity or solubility, thereby preventing precipitation and flocculation; The active groups of xanthan gum exchange with cations such as Ca2⁺ and Na⁺ on the surface of soil particles. Meanwhile, Cu(Ⅱ) -xanthan gum complexes are adsorbed onto the surface of soil particles through electrostatic interaction. Form a stable interface layer of soil particles—complexes—xanthan gum. Further enhance the connection strength between particles29. (2) reduced soil porosity results in a stronger soil skeleton; and (3) as the reaction progresses, the total amount of reaction products increases, most of the XG participates in the curing process, and the soil becomes more effectively cemented, leading to further strength improvement30.

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Relationship between UCS and XG content under freeze–thaw cycles conditions.

As shown in Fig. 9, the relationship between the compressive strength of cured soil specimens and XG content remains consistent regardless of freeze–thaw cycling. In both cases, the strength increases with higher XG content, exhibiting an approximately linear correlation. For specimens not subjected to freeze–thaw cycles, the strength increases steadily with increasing XG dosage, resembling a linear growth pattern. For specimens subjected to freeze–thaw cycles, the compressive strength also increases with both the number of cycles and the XG content, with the maximum strength gain becoming more pronounced at higher XG levels. This phenomenon can be attributed to the strong hydrophilicity of XG, which enhances the structural integrity of the soil matrix while simultaneously reducing pore space. Therefore, for the xanthan gum solidified soil in this study, a more stable water migration path is formed during freeze–thaw cycles, thereby mitigating the disruptive effects of water phase changes within the soil structure.

Effect of number of freeze–thaw cycles on strength

Figure 10 illustrates the variation in compressive strength of the cured soil as a function of the number of freeze–thaw cycles.

Fig. 10
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Relationship between UCS and number of freeze–thaw cycles.

As shown in Fig. 10, after undergoing freeze–thaw cycles, the compressive strength of specimens with XG contents of 0%, 1%, 2%, and 3% exhibits a biphasic trend: it increases with the number of cycles when the cycle count is below three, and decreases when the number exceeds three. This trend can be attributed to the microstructural evolution within the soil matrix, primarily driven by the bonding between the curing agent, contaminated soil, sediment, and soil particles, which collectively form a stable soil skeleton. During the freezing phase, liquid and gaseous water within the soil pores are subjected to low temperatures, leading to ice formation and associated volumetric expansion. This expansion generates internal stresses that compress and fragment the surrounding soil particles, thereby increasing pore volume and causing microstructural damage that compromises the integrity of the specimen. Consequently, the macroscopic strength of the specimen decreases. Analysis of the strength variation trend in Fig. 10 reveals that when the number of freeze–thaw cycles is less than six, the effect of freeze–thaw action on the strength of the stabilized soil is significant. However, beyond six cycles, the influence weakens due to the development of a more stable internal pore structure. Initially, the phase change of water during freezing causes structural damage to the pore network. During thawing, melted ice flows into newly formed pores and cracks. With repeated freeze–thaw cycles, water gradually establishes stable migration pathways, and the mechanical effects of water phase changes and frost heave forces on the soil structure tend to stabilize, resulting in a leveling off of the strength degradation trend.

Microstructural analysis of XG cured heavy metal contaminated soil

To understand the macroscopic mechanical behavior of soils, it is essential to consider the factors influencing their microstructural characteristics. In other words, the macroscopic behavior of soil is inherently dependent on specific microstructural features. The mechanical properties of soil are closely linked to its formation process, and these properties can vary significantly under different environmental conditions, indicating a strong and inseparable correlation between soil structure and its mechanical response. To fully comprehend the nature of this macro-mechanical behavior, it is necessary to investigate the soil’s microstructure, and laboratory testing serves as an effective method for obtaining relevant macro-mechanical parameters. In soil structure research, the concepts of macrostructure and microstructure can be studied independently, as proposed by Morgenstern. Microstructural analysis is particularly valuable for addressing practical engineering challenges and for providing foundational macroscopic parameters. Furthermore, the investigation of microstructure aims to establish the fundamental principles underlying mechanical behavior. This study focuses on in-depth analysis of microstructural features to develop a theoretical basis for understanding and predicting soil mechanics.

Analysis of SEM test results

Effect of XG content on microstructure

To investigate the influence of XG content on the microstructural characteristics of cured Cu(Ⅱ)-contaminated soil, scanning electron microscopy (SEM) was conducted on specimens with XG contents of 0% and 1%. The key to a representative analysis of graded discontinuous granular soil composed of coarse particles, fine particles and clay particles lies in matching the observation scale with the inherent heterogeneity of the material31. The initial selection target of the SEM samples for this test was to focus on the microstructure characteristics of the fine-grained soil matrix. Therefore, the sampling area was prioritized to be selected from the parts where the fine-grained content in the soil was relatively enriched. The initial selection target of the SEM samples for this test was to focus on the microstructure characteristics of the fine-grained soil matrix. Therefore, the sampling area was preferentially selected from the parts where the content of fine particles in the soil was relatively abundant. In the SEM experiment, we selected two magnification ratios of 2000 × and 10000 × . 2000 × was used to observe the overall arrangement of particles and pore distribution.10000 × was used to observe the cementation details between fine particles.

Figure 11 presents the microstructural characteristics of XG-treated Cu(Ⅱ)-contaminated soil. Figure 11a and b display the microstructure of untreated soil at magnifications of 2000 × and 10,000 × , respectively, while Fig. 11c and d show the microstructural features of soil treated with 1% XG at the same magnifications. As observed in Fig. 11c and d, the addition of XG resulted in the formation of a white colloidal substance on the surface of the specimen. These white membranes are mainly biofilms formed by the gelation products of a portion of xanthan gum covering the surface of soil particles. It can effectively bond soil particles together. After XG cements fine particles, a "fine particle-XG film complex" is formed. Transform the form of contact from “point contact” to “surface contact”. Figure 11 also demonstrates that the pores between soil particles gradually decrease and become more compact as the gelling substance fills the voids. Some xanthan gum fills the gaps between soil particles32,33. With coarse particles as the framework, its stability depends on the contact at the contact state points. XG indirectly enhances the load-bearing capacity of the framework by optimizing the cementation effect of fine particles. Bridge chains are composed of biopolymer chains. Under the biological polymerization chain of xanthan gum and the physical filling effect, soil particles are more closely connected. This makes the structure more solid34. This further explains the reason that the UCS of the sample gradually increases as the XG content increases.

Fig. 11
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Effect of XG on microstructure of contaminated soil.

The sheath of xanthan gum preferentially coats fine particles, forming a flexible “adhesive shell”. The coarse particles form a rigid "load-bearing chain" through point-surface contact. It is the coupling of the two that determines the final skeletal structure. Xanthan gum forms a flexible sheath on the fine-grained surface, providing adhesion. Coarse particles form a rigid force chain through point-surface contact, dominating friction and shear swelling. The two are coupled in a gradient along the particle size35.

Effect of freeze–thaw cycles on microstructure

Figure 12a and b present the scanning electron microscopy (SEM) images of specimens containing 1% XG after 0 and 6 freeze–thaw cycles, respectively, at a magnification of 10,000 × .

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Effect of freeze–thaw cycles on the microstructural morphology of solidified contaminated soil.

As shown in Fig. 12a, the soil particles are tightly wrapped by the cementing matrix of xanthan gum. It has no obvious large pores or cracks, forming a continuous and dense microstructure. The "cement-encapsulation" effect of xanthan gum is obvious, and the soil particles are closely combined with the cementing material36. The overall state is stable and solidified. After freeze–thaw cycles, the originally dense cemented matrix showed obvious cracks and damages. The number and size of the pores increased significantly, presenting a state of “pore development”. During the freeze–thaw cycle, the water in the soil constantly undergoes repeated actions of freezing expansion and melting migration37. This will disrupt the cementing structure between xanthan gum and soil particles, leading to the expansion of microscopic pores and the loosening of the structure. Therefore, the mechanical strength properties of the solidified soil decline after freeze–thaw cycles.

Limitations and challenges

The conclusions drawn from this study are limited to the tested materials, pollution levels and experimental conditions. In this study, only the specific concentration of 10,000 mg/kg was selected for research. Further studies can be conducted on the variation patterns of solidified soil at different concentrations. In subsequent work, further in-depth research on this study can be conducted. For instance, by integrating chemical or mineralogical characterizations such as XRD, FTIR or leaching tests, quantitative characterization and verification of the research results can be carried out.

The strength of the samples with the longest curing age of only 12 days in this study has not yet resolved the long-term strength variation. Future research should extend the curing age of soil samples. Strength tests and durability tests can be conducted within one month, three months or even longer to assess the long-term mechanical properties of the soil by the solidified material.

Conclusion

In this study, a series of laboratory experiments were conducted to investigate the effects of varying curing ages and XG contents on the mechanical properties of Cu(Ⅱ)-contaminated soil treated with XG. Furthermore, the changes in physical and mechanical properties of the stabilized soil specimens under repeated freeze–thaw cycles were examined. The key findings of this study are summarized as follows:

As the curing age increases, the strength of the stabilized soil shows a progressive upward trend, although the rate of increase gradually diminishes. The stress-strain curves of the cured soil at different curing ages can be classified into three distinct stages: the compression-densification stage, the elastic deformation stage, and the failure stage. With prolonged curing, peak stress increases while destructive straindecreases. Consequently, the stress-strain curves exhibit sharper rises and abrupt drops, indicating an enhancement in the brittleness of the cured soil.

Scanning electron microscopy (SEM) analysis of the cured soil containing 1% XG after six days of standard curing and freeze-thaw cycling revealed that the strength development primarily results from the improved interaction between the curing agent and copper ions. As the XG content increases, the microstructure becomes increasingly compact, thereby enhancing the structural integrity of the soil.

Freeze-thaw cycles tests conducted on specimens cured for six days under standard conditions indicate that the strength of the specimens decreases with an increasing number of cycles. However, this reduction in strength stabilizes as the number of cycles increases.

The deformation behavior of the specimens was analyzed based on the freeze-thaw test results. The analysis showed that the stress-strain curve can be divided into four distinct stages. Moreover, it was observed that the peak stress decreases while the destructive strain increases with the number of freeze-thaw cycles.